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        <div>
            <div><h3 class="title">
                <a name="asio.reference.asynchronous_operations"></a><a class="link" href="asynchronous_operations.html"
                                                                        title="Requirements on asynchronous operations">Requirements
                on asynchronous operations</a>
            </h3></div>
        </div>
    </div>
    <p>
        This section uses the names <code class="computeroutput"><span class="identifier">Alloc1</span></code>,
        <code class="computeroutput"><span class="identifier">Alloc2</span></code>, <code class="computeroutput"><span
            class="identifier">alloc1</span></code>,
        <code class="computeroutput"><span class="identifier">alloc2</span></code>, <code class="computeroutput"><span
            class="identifier">Args</span></code>,
        <code class="computeroutput"><span class="identifier">CompletionHandler</span></code>, <code
            class="computeroutput"><span class="identifier">completion_handler</span></code>, <code
            class="computeroutput"><span class="identifier">Executor1</span></code>,
        <code class="computeroutput"><span class="identifier">Executor2</span></code>, <code
            class="computeroutput"><span class="identifier">ex1</span></code>, <code class="computeroutput"><span
            class="identifier">ex2</span></code>,
        <code class="computeroutput"><span class="identifier">f</span></code>, <code class="literal"><span
            class="emphasis"><em>i</em></span></code>,
        <code class="literal"><span class="emphasis"><em>N</em></span></code>, <code class="computeroutput"><span
            class="identifier">Signatures</span></code>,
        <code class="computeroutput"><span class="identifier">token</span></code>, <code class="computeroutput"><span
            class="identifier">initiation</span></code>,
        <code class="computeroutput"><span class="identifier">Initiation</span></code>, <code class="literal">T[sub
        <span class="emphasis"><em>i</em></span>]</code>, <code class="literal">t[sub <span class="emphasis"><em>i</em></span>]</code>,
        <code class="computeroutput"><span class="identifier">work1</span></code>, and <code
            class="computeroutput"><span class="identifier">work2</span></code> as placeholders for specifying the
        requirements below.
    </p>
    <h5>
        <a name="asio.reference.asynchronous_operations.h0"></a>
        <span><a name="asio.reference.asynchronous_operations.general_asynchronous_operation_concepts"></a></span><a
            class="link"
            href="asynchronous_operations.html#asio.reference.asynchronous_operations.general_asynchronous_operation_concepts">General
        asynchronous operation concepts</a>
    </h5>
    <p>
        An <span class="emphasis"><em>initiating function</em></span> is a function which may be called
        to start an asynchronous operation. A <span class="emphasis"><em>completion handler</em></span>
        is a function object that will be invoked, at most once, with the result
        of the asynchronous operation.
    </p>
    <p>
        The lifecycle of an asynchronous operation is comprised of the following
        events and phases:
    </p>
    <p>
        — Event 1: The asynchronous operation is started by a call to the initiating
        function.
    </p>
    <p>
        — Phase 1: The asynchronous operation is now <span class="emphasis"><em>outstanding</em></span>.
    </p>
    <p>
        — Event 2: The externally observable side effects of the asynchronous operation,
        if any, are fully established. The completion handler is submitted to an
        executor.
    </p>
    <p>
        — Phase 2: The asynchronous operation is now <span class="emphasis"><em>completed</em></span>.
    </p>
    <p>
        — Event 3: The completion handler is called with the result of the asynchronous
        operation.
    </p>
    <p>
        In this library, all functions with the prefix <code class="computeroutput"><span
            class="identifier">async_</span></code>
        are initiating functions.
    </p>
    <h5>
        <a name="asio.reference.asynchronous_operations.h1"></a>
        <span><a name="asio.reference.asynchronous_operations.completion_tokens_and_handlers"></a></span><a class="link"
                                                                                                            href="asynchronous_operations.html#asio.reference.asynchronous_operations.completion_tokens_and_handlers">Completion
        tokens and handlers</a>
    </h5>
    <p>
        Initiating functions:
    </p>
    <p>
        — are function templates with template parameter <code class="computeroutput"><span class="identifier">CompletionToken</span></code>;
    </p>
    <p>
        — accept, as the final parameter, a <span class="emphasis"><em>completion token</em></span> object
        <code class="computeroutput"><span class="identifier">token</span></code> of type <code
            class="computeroutput"><span class="identifier">CompletionToken</span></code>;
    </p>
    <p>
        — specify one or more <span class="emphasis"><em>completion signatures</em></span>, referred
        to below as a variadic pack of call signatures (C++Std [func.def]) <code class="computeroutput"><span
            class="identifier">Signatures</span></code>, that determine the possible
        argument sets to the completion handler.
    </p>
    <p>
        In this library, initiating functions specify a <span class="emphasis"><em>Completion signature</em></span>
        element that defines the call signatures <code class="computeroutput"><span class="identifier">Signatures</span></code>.
        The <span class="emphasis"><em>Completion signature</em></span> elements in this library may
        have named parameters, and the results of an asynchronous operation may be
        specified in terms of these names.
    </p>
    <p>
        Completion token behaviour is determined through specialisation of the <code class="computeroutput"><span
            class="identifier">async_result</span></code> trait. These specialisations
        must have the form:
    </p>
    <pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span
            class="keyword">class</span> <span class="identifier">CompletionToken</span><span
            class="special">,</span> <span class="keyword">class</span><span class="special">...</span> <span
            class="identifier">Signatures</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">async_result</span>
<span class="special">{</span>
  <span class="keyword">template</span><span class="special">&lt;</span>
      <span class="keyword">class</span> <span class="identifier">Initiation</span><span class="special">,</span>
      <span class="keyword">class</span> <span class="identifier">RawCompletionToken</span><span
                class="special">,</span>
      <span class="keyword">class</span><span class="special">...</span> <span class="identifier">Args</span>
    <span class="special">&gt;</span>
  <span class="keyword">static</span> <span class="identifier">initiating</span><span class="special">-</span><span
                class="identifier">fn</span><span class="special">-</span><span class="keyword">return</span><span
                class="special">-</span><span class="identifier">type</span> <span
                class="identifier">initiate</span><span class="special">(</span>
      <span class="identifier">Initiation</span><span class="special">&amp;&amp;</span> <span class="identifier">initiation</span><span
                class="special">,</span>
      <span class="identifier">RawCompletionToken</span><span class="special">&amp;&amp;</span> <span
                class="identifier">token</span><span class="special">,</span>
      <span class="identifier">Args</span><span class="special">&amp;&amp;...</span> <span
                class="identifier">args</span>
    <span class="special">);</span>
<span class="special">};</span>
</pre>
    <p>
        An <code class="computeroutput"><span class="identifier">async_result</span></code> specialisation’s
        implementation of the <code class="computeroutput"><span class="identifier">initiate</span><span
            class="special">()</span></code> static member function must:
    </p>
    <div class="itemizedlist">
        <ul class="itemizedlist" type="disc">
            <li class="listitem">
                Transform the completion token into an object <code class="computeroutput"><span class="identifier">completion_handler</span></code>
                of type <code class="computeroutput"><span class="identifier">CompletionHandler</span></code>.
                The type <code class="computeroutput"><span class="identifier">CompletionHandler</span></code>
                must satisfy the requirements of <code class="computeroutput"><span
                    class="identifier">Destructible</span></code>
                (C++Std [destructible]) and <code class="computeroutput"><span
                    class="identifier">MoveConstructible</span></code>
                (C++Std [moveconstructible]), and be callable with the specified call
                signatures.
            </li>
            <li class="listitem">
                <p class="simpara">
                    Cause the invocation of the function object <code class="computeroutput"><span class="identifier">initiation</span></code>
                    as if by calling:
                </p>
                <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span
                        class="identifier">forward</span><span class="special">&lt;</span><span class="identifier">Initiation</span><span
                        class="special">&gt;(</span><span class="identifier">initiation</span><span
                        class="special">)(</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span
                            class="special">(</span><span class="identifier">completion_handler</span><span
                            class="special">),</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">forward</span><span
                            class="special">&lt;</span><span class="identifier">Args</span><span
                            class="special">&gt;(</span><span class="identifier">args</span><span
                            class="special">)...);</span>
</pre>
            </li>
        </ul>
    </div>
    <p>
        This invocation of <code class="computeroutput"><span class="identifier">initiation</span></code>
        may be immediate, or it may be deferred (e.g. to support lazy evaluation).
        If <code class="computeroutput"><span class="identifier">initiation</span></code> is deferred,
        the <code class="computeroutput"><span class="identifier">initiation</span></code> and <code
            class="computeroutput"><span class="identifier">args</span><span class="special">...</span></code>
        objects must be decay-copied and moved as required.
    </p>
    <p>
        The <code class="computeroutput"><span class="identifier">async_result</span></code> trait must
        be specialised for the decayed type of a <code class="computeroutput"><span
            class="identifier">CompletionToken</span></code>.
        A helper function template <code class="computeroutput"><span class="identifier">async_initiate</span></code>
        is provided to simplify correct invocation of <code class="computeroutput"><span
            class="identifier">async_result</span><span class="special">&lt;&gt;::</span><span class="identifier">initiate</span></code>
        for the appropriate specialisation:
    </p>
    <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span>
    <span class="keyword">class</span> <span class="identifier">CompletionToken</span><span class="special">,</span>
    <span class="identifier">completion_signature</span><span class="special">...</span> <span class="identifier">Signatures</span><span
                class="special">,</span>
    <span class="keyword">class</span> <span class="identifier">Initiation</span><span class="special">,</span>
    <span class="keyword">class</span><span class="special">...</span> <span class="identifier">Args</span>
  <span class="special">&gt;</span>
<span class="emphasis"><em>DEDUCED</em></span> <span class="identifier">async_initiate</span><span
                class="special">(</span>
    <span class="identifier">Initiation</span><span class="special">&amp;&amp;</span> <span class="identifier">initiation</span><span
                class="special">,</span>
    <span class="identifier">CompletionToken</span><span class="special">&amp;</span> <span
                class="identifier">token</span><span class="special">,</span>
    <span class="identifier">Args</span><span class="special">&amp;&amp;...</span> <span class="identifier">args</span>
  <span class="special">);</span>
</pre>
    <p>
        [<span class="emphasis"><em>Note:</em></span> No other requirements are placed on the type
        <code class="computeroutput"><span class="identifier">CompletionToken</span></code>. —<span
            class="emphasis"><em>end
        note</em></span>]
    </p>
    <h5>
        <a name="asio.reference.asynchronous_operations.h2"></a>
        <span><a
                name="asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type"></a></span><a
            class="link"
            href="asynchronous_operations.html#asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type">Automatic
        deduction of initiating function return type</a>
    </h5>
    <p>
        An initiating function returns <code class="literal">async_initiate&lt;CompletionToken,
        Signatures...&gt;(initiation, token, <span class="emphasis"><em>unspecified-args...</em></span>)</code>,
        where <code class="computeroutput"><span class="identifier">initiation</span></code> is a function
        object of unspecified type <code class="computeroutput"><span class="identifier">Initiation</span></code>,
        which is defined as:
    </p>
    <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">Initiation</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
  <span class="keyword">using</span> <span class="identifier">executor_type</span> <span class="special">=</span> <span
                class="identifier">Executor</span><span class="special">;</span> <span
                class="comment">// optional</span>
  <span class="identifier">executor_type</span> <span class="identifier">get_executor</span><span
                class="special">()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span><span
                class="special">;</span> <span class="comment">// optional</span>

  <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span
                class="identifier">CompletionHandler</span><span class="special">,</span> <span class="emphasis"><em>unspecified-args...</em></span><span
                class="special">&gt;</span>
    <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span
                class="identifier">CompletionHandler</span><span class="special">&amp;&amp;</span> <span
                class="identifier">completion_handler</span><span class="special">,</span> <span class="emphasis"><em>unspecified-args...</em></span><span
                class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
<span class="special">};</span>
</pre>
    <p>
        For the sake of exposition, this library sometimes annotates functions with
        a return type <span class="emphasis"><em><code class="literal">DEDUCED</code></em></span>. For every
        function declaration that returns <span class="emphasis"><em><code class="literal">DEDUCED</code></em></span>,
        the meaning is equivalent to an automatically deduced return type, having
        the type of the expression <code class="literal">async_initiate&lt;CompletionToken, Signatures...&gt;(initiation,
        token, <span class="emphasis"><em>unspecified-args...</em></span>)</code>.
    </p>
    <p>
        [<span class="emphasis"><em>Example:</em></span> Given an asynchronous operation with a single
        <span class="emphasis"><em>Completion signature</em></span> <code class="computeroutput"><span class="keyword">void</span><span
            class="special">(</span><span class="identifier">R1</span> <span class="identifier">r1</span><span
            class="special">,</span> <span class="identifier">R2</span> <span class="identifier">r2</span><span
            class="special">)</span></code>, an initiating function meeting these requirements
        may be implemented as follows:
    </p>
    <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span
            class="keyword">class</span> <span class="identifier">CompletionToken</span><span
            class="special">&gt;</span>
<span class="keyword">auto</span> <span class="identifier">async_xyz</span><span class="special">(</span><span
                class="identifier">T1</span> <span class="identifier">t1</span><span class="special">,</span> <span
                class="identifier">T2</span> <span class="identifier">t2</span><span class="special">,</span> <span
                class="identifier">CompletionToken</span><span class="special">&amp;&amp;</span> <span
                class="identifier">token</span><span class="special">)</span>
<span class="special">{</span>
  <span class="keyword">return</span> <span class="identifier">async_initiate</span><span
                class="special">&lt;</span><span class="identifier">CompletionToken</span><span class="special">,</span> <span
                class="keyword">void</span><span class="special">(</span><span class="identifier">R1</span><span
                class="special">,</span> <span class="identifier">R2</span><span class="special">)&gt;(</span>
      <span class="special">[](</span><span class="keyword">auto</span> <span
                class="identifier">completion_handler</span><span class="special">,</span> <span
                class="identifier">T1</span> <span class="identifier">t1</span><span class="special">,</span> <span
                class="identifier">T2</span> <span class="identifier">t2</span><span class="special">)</span>
      <span class="special">{</span>
        <span class="comment">// initiate the operation and cause completion_handler to be invoked</span>
        <span class="comment">// with the result</span>
      <span class="special">},</span> <span class="identifier">token</span><span class="special">,</span> <span
                class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span
                class="special">(</span><span class="identifier">t1</span><span class="special">),</span> <span
                class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span
                class="special">(</span><span class="identifier">t2</span><span class="special">));</span>
<span class="special">}</span>
</pre>
    <p>
        The concepts <code class="computeroutput"><span class="identifier">completion_token_for</span></code>
        and <code class="computeroutput"><span class="identifier">completion_handler_for</span></code>
        may also be used to improve compile-time diagnostics:
    </p>
    <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span
            class="identifier">completion_token_for</span><span class="special">&lt;</span><span
            class="keyword">void</span><span class="special">(</span><span class="identifier">R1</span><span
            class="special">,</span> <span class="identifier">R2</span><span class="special">)&gt;</span> <span
            class="identifier">CompletionToken</span><span class="special">&gt;</span>
<span class="keyword">auto</span> <span class="identifier">async_xyz</span><span class="special">(</span><span
                class="identifier">T1</span> <span class="identifier">t1</span><span class="special">,</span> <span
                class="identifier">T2</span> <span class="identifier">t2</span><span class="special">,</span> <span
                class="identifier">CompletionToken</span><span class="special">&amp;&amp;</span> <span
                class="identifier">token</span><span class="special">)</span>
<span class="special">{</span>
  <span class="keyword">return</span> <span class="identifier">async_initiate</span><span
                class="special">&lt;</span><span class="identifier">CompletionToken</span><span class="special">,</span> <span
                class="keyword">void</span><span class="special">(</span><span class="identifier">R1</span><span
                class="special">,</span> <span class="identifier">R2</span><span class="special">)&gt;(</span>
      <span class="special">[](</span><span class="identifier">completion_handler_for</span><span
                class="special">&lt;</span><span class="keyword">void</span><span class="special">(</span><span
                class="identifier">R1</span><span class="special">,</span> <span class="identifier">R2</span><span
                class="special">)&gt;</span> <span class="keyword">auto</span> <span class="identifier">completion_handler</span><span
                class="special">,</span> <span class="identifier">T1</span> <span class="identifier">t1</span><span
                class="special">,</span> <span class="identifier">T2</span> <span class="identifier">t2</span><span
                class="special">)</span>
      <span class="special">{</span>
        <span class="comment">// initiate the operation and cause completion_handler to be invoked</span>
        <span class="comment">// with the result</span>
      <span class="special">},</span> <span class="identifier">token</span><span class="special">,</span> <span
                class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span
                class="special">(</span><span class="identifier">t1</span><span class="special">),</span> <span
                class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span
                class="special">(</span><span class="identifier">t2</span><span class="special">));</span>
<span class="special">}</span>
</pre>
    <p>
        Initiation functions may also be implemented using the <code class="computeroutput"><span class="identifier">async_result</span></code>
        trait directly:
    </p>
    <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span
            class="keyword">class</span> <span class="identifier">CompletionToken</span><span
            class="special">&gt;</span>
<span class="keyword">auto</span> <span class="identifier">async_xyz</span><span class="special">(</span><span
                class="identifier">T1</span> <span class="identifier">t1</span><span class="special">,</span> <span
                class="identifier">T2</span> <span class="identifier">t2</span><span class="special">,</span> <span
                class="identifier">CompletionToken</span><span class="special">&amp;&amp;</span> <span
                class="identifier">token</span><span class="special">)</span>
<span class="special">{</span>
  <span class="keyword">return</span> <span class="identifier">async_result</span><span class="special">&lt;</span><span
                class="identifier">decay_t</span><span class="special">&lt;</span><span class="identifier">CompletionToken</span><span
                class="special">&gt;,</span> <span class="keyword">void</span><span class="special">(</span><span
                class="identifier">R1</span><span class="special">,</span> <span class="identifier">R2</span><span
                class="special">)&gt;::</span><span class="identifier">initiate</span><span class="special">(</span>
      <span class="special">[](</span><span class="keyword">auto</span> <span
                class="identifier">completion_handler</span><span class="special">,</span> <span
                class="identifier">T1</span> <span class="identifier">t1</span><span class="special">,</span> <span
                class="identifier">T2</span> <span class="identifier">t2</span><span class="special">)</span>
      <span class="special">{</span>
        <span class="comment">// initiate the operation and cause completion_handler to be invoked</span>
        <span class="comment">// with the result</span>
      <span class="special">},</span> <span class="identifier">forward</span><span class="special">&lt;</span><span
                class="identifier">CompletionToken</span><span class="special">&gt;(</span><span class="identifier">token</span><span
                class="special">),</span> <span class="identifier">std</span><span class="special">::</span><span
                class="identifier">move</span><span class="special">(</span><span class="identifier">t1</span><span
                class="special">),</span> <span class="identifier">std</span><span class="special">::</span><span
                class="identifier">move</span><span class="special">(</span><span class="identifier">t2</span><span
                class="special">));</span>
<span class="special">}</span>
</pre>
    <p>
        Note the use of <code class="computeroutput"><span class="identifier">decay_t</span></code> and
        <code class="computeroutput"><span class="identifier">forward</span></code> applied to the <code
            class="computeroutput"><span class="identifier">CompletionToken</span></code> type. However, the first
        form is preferred as it preserves compatibility with legacy completion token
        requirements.
    </p>
    <p>
        —<span class="emphasis"><em>end example</em></span>]
    </p>
    <h5>
        <a name="asio.reference.asynchronous_operations.h3"></a>
        <span><a name="asio.reference.asynchronous_operations.lifetime_of_initiating_function_arguments"></a></span><a
            class="link"
            href="asynchronous_operations.html#asio.reference.asynchronous_operations.lifetime_of_initiating_function_arguments">Lifetime
        of initiating function arguments</a>
    </h5>
    <p>
        Unless otherwise specified, the lifetime of arguments to initiating functions
        shall be treated as follows:
    </p>
    <p>
        — If the parameter has a pointer type or has a type of lvalue reference to
        non-const, the implementation may assume the validity of the pointee or referent,
        respectively, until the completion handler is invoked. [<span class="emphasis"><em>Note:</em></span>
        In other words, the program must guarantee the validity of the argument until
        the completion handler is invoked. —<span class="emphasis"><em>end note</em></span>]
    </p>
    <p>
        — Otherwise, the implementation must not assume the validity of the argument
        after the initiating function completes. [<span class="emphasis"><em>Note:</em></span> In other
        words, the program is not required to guarantee the validity of the argument
        after the initiating function completes. —<span class="emphasis"><em>end note</em></span>] The
        implementation may make copies of the argument, and all copies shall be destroyed
        no later than immediately after invocation of the completion handler.
    </p>
    <h5>
        <a name="asio.reference.asynchronous_operations.h4"></a>
        <span><a
                name="asio.reference.asynchronous_operations.non_blocking_requirements_on_initiating_functions"></a></span><a
            class="link"
            href="asynchronous_operations.html#asio.reference.asynchronous_operations.non_blocking_requirements_on_initiating_functions">Non-blocking
        requirements on initiating functions</a>
    </h5>
    <p>
        An initiating function shall not block (C++Std [defns.block]) the calling
        thread pending completion of the outstanding operation.
    </p>
    <p>
        [<span class="emphasis"><em>Note:</em></span> Initiating functions may still block the calling
        thread for other reasons. For example, an initiating function may lock a
        mutex in order to synchronize access to shared data. —<span class="emphasis"><em>end note</em></span>]
    </p>
    <h5>
        <a name="asio.reference.asynchronous_operations.h5"></a>
        <span><a name="asio.reference.asynchronous_operations.associated_executor"></a></span><a class="link"
                                                                                                 href="asynchronous_operations.html#asio.reference.asynchronous_operations.associated_executor">Associated
        executor</a>
    </h5>
    <p>
        Certain objects that participate in asynchronous operations have an <span class="emphasis"><em>associated
        executor</em></span>. These are obtained as specified in the sections below.
    </p>
    <h5>
        <a name="asio.reference.asynchronous_operations.h6"></a>
        <span><a name="asio.reference.asynchronous_operations.associated_i_o_executor"></a></span><a class="link"
                                                                                                     href="asynchronous_operations.html#asio.reference.asynchronous_operations.associated_i_o_executor">Associated
        I/O executor</a>
    </h5>
    <p>
        An asynchronous operation has an associated executor satisfying the <a class="link" href="Executor1.html"
                                                                               title="Executor requirements"><code
            class="computeroutput"><span class="identifier">Executor</span></code></a>
        requirements. If not otherwise specified by the asynchronous operation, this
        associated executor is an object of type <code class="computeroutput"><span
            class="identifier">system_executor</span></code>.
    </p>
    <p>
        All asynchronous operations in this library have an associated executor object
        that is determined as follows:
    </p>
    <p>
        — If the initiating function is a member function, the associated executor
        is that returned by the <code class="computeroutput"><span class="identifier">get_executor</span></code>
        member function on the same object.
    </p>
    <p>
        — If the initiating function is not a member function, the associated executor
        is that returned by the <code class="computeroutput"><span class="identifier">get_executor</span></code>
        member function of the first argument to the initiating function.
    </p>
    <p>
        The operation's associated I/O executor may be exposed via the <code class="computeroutput"><span
            class="identifier">Initiation</span></code> function object's <code class="computeroutput"><span
            class="identifier">executor_type</span></code> type alias and <code class="computeroutput"><span
            class="identifier">get_executor</span><span class="special">()</span></code>
        member function.
    </p>
    <p>
        Let <code class="computeroutput"><span class="identifier">Executor1</span></code> be the type
        of the associated executor. Let <code class="computeroutput"><span class="identifier">ex1</span></code>
        be a value of type <code class="computeroutput"><span class="identifier">Executor1</span></code>,
        representing the associated executor object obtained as described above.
    </p>
    <h5>
        <a name="asio.reference.asynchronous_operations.h7"></a>
        <span><a name="asio.reference.asynchronous_operations.associated_completion_handler_executor"></a></span><a
            class="link"
            href="asynchronous_operations.html#asio.reference.asynchronous_operations.associated_completion_handler_executor">Associated
        completion handler executor</a>
    </h5>
    <p>
        A completion handler object of type <code class="computeroutput"><span
            class="identifier">CompletionHandler</span></code>
        has an associated executor of type <code class="computeroutput"><span class="identifier">Executor2</span></code>
        satisfying the <a class="link" href="Executor1.html" title="Executor requirements">Executor requirements</a>.
        The type <code class="computeroutput"><span class="identifier">Executor2</span></code> is <code
            class="computeroutput"><span class="identifier">associated_executor_t</span><span
            class="special">&lt;</span><span class="identifier">CompletionHandler</span><span class="special">,</span>
        <span class="identifier">Executor1</span><span class="special">&gt;</span></code>.
        Let <code class="computeroutput"><span class="identifier">ex2</span></code> be a value of type
        <code class="computeroutput"><span class="identifier">Executor2</span></code> obtained by performing
        <code class="computeroutput"><span class="identifier">get_associated_executor</span><span
                class="special">(</span><span class="identifier">completion_handler</span><span class="special">,</span>
            <span class="identifier">ex1</span><span class="special">)</span></code>.
    </p>
    <h5>
        <a name="asio.reference.asynchronous_operations.h8"></a>
        <span><a name="asio.reference.asynchronous_operations.outstanding_work"></a></span><a class="link"
                                                                                              href="asynchronous_operations.html#asio.reference.asynchronous_operations.outstanding_work">Outstanding
        work</a>
    </h5>
    <p>
        Until the asynchronous operation has completed, the asynchronous operation
        shall maintain:
    </p>
    <p>
        — an object <code class="computeroutput"><span class="identifier">work1</span></code> of type
        <code class="computeroutput"><span class="identifier">executor_work_guard</span><span
                class="special">&lt;</span><span class="identifier">Executor1</span><span
                class="special">&gt;</span></code>,
        initialized as <code class="computeroutput"><span class="identifier">work1</span><span
            class="special">(</span><span class="identifier">ex1</span><span class="special">)</span></code>, and
        where <code class="computeroutput"><span class="identifier">work1</span><span class="special">.</span><span
            class="identifier">owns_work</span><span class="special">()</span> <span class="special">==</span> <span
            class="keyword">true</span></code>; and
    </p>
    <p>
        — an object <code class="computeroutput"><span class="identifier">work2</span></code> of type
        <code class="computeroutput"><span class="identifier">executor_work_guard</span><span
                class="special">&lt;</span><span class="identifier">Executor2</span><span
                class="special">&gt;</span></code>,
        initialized as <code class="computeroutput"><span class="identifier">work2</span><span
            class="special">(</span><span class="identifier">ex2</span><span class="special">)</span></code>, and
        where <code class="computeroutput"><span class="identifier">work2</span><span class="special">.</span><span
            class="identifier">owns_work</span><span class="special">()</span> <span class="special">==</span> <span
            class="keyword">true</span></code>.
    </p>
    <h5>
        <a name="asio.reference.asynchronous_operations.h9"></a>
        <span><a name="asio.reference.asynchronous_operations.allocation_of_intermediate_storage"></a></span><a
            class="link"
            href="asynchronous_operations.html#asio.reference.asynchronous_operations.allocation_of_intermediate_storage">Allocation
        of intermediate storage</a>
    </h5>
    <p>
        Asynchronous operations may allocate memory. [<span class="emphasis"><em>Note:</em></span>
        Such as a data structure to store copies of the <code class="computeroutput"><span class="identifier">completion_handler</span></code>
        object and the initiating function's arguments. —<span class="emphasis"><em>end note</em></span>]
    </p>
    <p>
        Let <code class="computeroutput"><span class="identifier">Alloc1</span></code> be a type, satisfying
        the <a class="link" href="ProtoAllocator.html" title="Proto-allocator requirements"><code
            class="computeroutput"><span class="identifier">ProtoAllocator</span></code></a>
        requirements, that represents the asynchronous operation's default allocation
        strategy. [<span class="emphasis"><em>Note:</em></span> Typically <code class="computeroutput"><span
            class="identifier">std</span><span class="special">::</span><span class="identifier">allocator</span><span
            class="special">&lt;</span><span class="keyword">void</span><span class="special">&gt;</span></code>.
        —<span class="emphasis"><em>end note</em></span>] Let <code class="computeroutput"><span class="identifier">alloc1</span></code>
        be a value of type <code class="computeroutput"><span class="identifier">Alloc1</span></code>.
    </p>
    <p>
        A completion handler object of type <code class="computeroutput"><span
            class="identifier">CompletionHandler</span></code>
        has an associated allocator object <code class="computeroutput"><span class="identifier">alloc2</span></code>
        of type <code class="computeroutput"><span class="identifier">Alloc2</span></code> satisfying
        the <a class="link" href="ProtoAllocator.html" title="Proto-allocator requirements"><code
            class="computeroutput"><span class="identifier">ProtoAllocator</span></code></a>
        requirements. The type <code class="computeroutput"><span class="identifier">Alloc2</span></code>
        is <code class="computeroutput"><span class="identifier">associated_allocator_t</span><span
            class="special">&lt;</span><span class="identifier">CompletionHandler</span><span class="special">,</span>
        <span class="identifier">Alloc1</span><span class="special">&gt;</span></code>. Let <code
            class="computeroutput"><span class="identifier">alloc2</span></code>
        be a value of type <code class="computeroutput"><span class="identifier">Alloc2</span></code>
        obtained by performing <code class="computeroutput"><span
            class="identifier">get_associated_allocator</span><span class="special">(</span><span class="identifier">completion_handler</span><span
            class="special">,</span> <span class="identifier">alloc1</span><span class="special">)</span></code>.
    </p>
    <p>
        The asynchronous operations defined in this library:
    </p>
    <p>
        — If required, allocate memory using only the completion handler's associated
        allocator.
    </p>
    <p>
        — Prior to completion handler execution, deallocate any memory allocated using
        the completion handler's associated allocator.
    </p>
    <p>
        [<span class="emphasis"><em>Note:</em></span> The implementation may perform operating system
        or underlying API calls that perform memory allocations not using the associated
        allocator. Invocations of the allocator functions may not introduce data
        races (See C++Std [res.on.data.races]). —<span class="emphasis"><em>end note</em></span>]
    </p>
    <h5>
        <a name="asio.reference.asynchronous_operations.h10"></a>
        <span><a
                name="asio.reference.asynchronous_operations.execution_of_completion_handler_on_completion_of_asynchronous_operation"></a></span><a
            class="link"
            href="asynchronous_operations.html#asio.reference.asynchronous_operations.execution_of_completion_handler_on_completion_of_asynchronous_operation">Execution
        of completion handler on completion of asynchronous operation</a>
    </h5>
    <p>
        Let <code class="computeroutput"><span class="identifier">Args</span><span class="special">...</span></code>
        be the argument types of a completion signature in <code class="computeroutput"><span class="identifier">Signatures</span><span
            class="special">...</span></code> and let <code class="literal"><span
            class="emphasis"><em>N</em></span></code>
        be <code class="computeroutput"><span class="keyword">sizeof</span><span class="special">...(</span><span
            class="identifier">Args</span><span class="special">)</span></code>.
        Let <code class="literal"><span class="emphasis"><em>i</em></span></code> be in the range [<code
            class="computeroutput"><span class="number">0</span></code>,<code class="literal"><span
            class="emphasis"><em>N</em></span></code>).
        Let <code class="literal">T[sub <span class="emphasis"><em>i</em></span>]</code> be the <code
            class="literal"><span class="emphasis"><em>i</em></span></code>th
        type in <code class="computeroutput"><span class="identifier">Args</span><span class="special">...</span></code>
        and let <code class="literal">t[sub <span class="emphasis"><em>i</em></span>]</code> be the <code
            class="literal"><span class="emphasis"><em>i</em></span></code>th
        completion handler argument associated with <code class="literal">T[sub <span class="emphasis"><em>i</em></span>]</code>.
    </p>
    <p>
        Let <code class="computeroutput"><span class="identifier">f</span></code> be a function object,
        callable as <code class="computeroutput"><span class="identifier">f</span><span class="special">()</span></code>,
        that invokes <code class="computeroutput"><span class="identifier">completion_handler</span></code>
        as if by <code class="literal">completion_handler(forward&lt;T[sub <span class="emphasis"><em>0</em></span>]&gt;(t[sub
        <span class="emphasis"><em>0</em></span>]), ..., forward&lt;T[sub <span class="emphasis"><em>N-1</em></span>]&gt;(t[sub
        <span class="emphasis"><em>N-1</em></span>]))</code>.
    </p>
    <p>
        If an asynchonous operation completes immediately (that is, within the thread
        of execution calling the initiating function, and before the initiating function
        returns), the completion handler shall be submitted for execution as if by
        performing <code class="computeroutput"><span class="identifier">post</span><span class="special">(</span><span
            class="identifier">ex2</span><span class="special">,</span> <span class="identifier">std</span><span
            class="special">::</span><span class="identifier">move</span><span class="special">(</span><span
            class="identifier">f</span><span class="special">),</span>
        <span class="identifier">alloc2</span><span class="special">)</span></code>.
        Otherwise, the completion handler shall be submitted for execution as if
        by performing <code class="computeroutput"><span class="identifier">dispatch</span><span
            class="special">(</span><span class="identifier">ex2</span><span class="special">,</span> <span
            class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span
            class="special">(</span><span class="identifier">f</span><span class="special">),</span>
        <span class="identifier">alloc2</span><span class="special">)</span></code>.
    </p>
    <h5>
        <a name="asio.reference.asynchronous_operations.h11"></a>
        <span><a name="asio.reference.asynchronous_operations.completion_handlers_and_exceptions"></a></span><a
            class="link"
            href="asynchronous_operations.html#asio.reference.asynchronous_operations.completion_handlers_and_exceptions">Completion
        handlers and exceptions</a>
    </h5>
    <p>
        Completion handlers are permitted to throw exceptions. The effect of any
        exception propagated from the execution of a completion handler is determined
        by the executor which is executing the completion handler.
    </p>
    <h5>
        <a name="asio.reference.asynchronous_operations.h12"></a>
        <span><a name="asio.reference.asynchronous_operations.default_completion_tokens"></a></span><a class="link"
                                                                                                       href="asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens">Default
        completion tokens</a>
    </h5>
    <p>
        Every I/O executor type has an associated default completion token type.
        This is specified via the <code class="computeroutput"><span class="identifier">default_completion_token</span></code>
        trait. This trait may be used in asynchronous operation declarations as follows:
    </p>
    <pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span>
    <span class="keyword">typename</span> <span class="identifier">IoObject</span><span class="special">,</span>
    <span class="keyword">typename</span> <span class="identifier">CompletionToken</span> <span class="special">=</span>
      <span class="keyword">typename</span> <span class="identifier">default_completion_token</span><span
                class="special">&lt;</span>
        <span class="keyword">typename</span> <span class="identifier">IoObject</span><span
                class="special">::</span><span class="identifier">executor_type</span>
      <span class="special">&gt;::</span><span class="identifier">type</span>
  <span class="special">&gt;</span>
<span class="keyword">auto</span> <span class="identifier">async_xyz</span><span class="special">(</span>
    <span class="identifier">IoObject</span><span class="special">&amp;</span> <span class="identifier">io_object</span><span
                class="special">,</span>
    <span class="identifier">CompletionToken</span><span class="special">&amp;&amp;</span> <span class="identifier">token</span> <span
                class="special">=</span>
      <span class="keyword">typename</span> <span class="identifier">default_completion_token</span><span
                class="special">&lt;</span>
        <span class="keyword">typename</span> <span class="identifier">IoObject</span><span
                class="special">::</span><span class="identifier">executor_type</span>
      <span class="special">&gt;::</span><span class="identifier">type</span><span class="special">{}</span>
  <span class="special">);</span>
</pre>
    <p>
        If not specialised, this trait type is <code class="computeroutput"><span class="keyword">void</span></code>,
        meaning no default completion token type is available for the given I/O executor.
    </p>
    <p>
        [<span class="emphasis"><em>Example:</em></span> The <code class="computeroutput"><span class="identifier">default_completion_token</span></code>
        trait is specialised for the <code class="computeroutput"><span class="identifier">use_awaitable</span></code>
        completion token so that it may be used as shown in the following example:
    </p>
    <pre class="programlisting"><span class="keyword">auto</span> <span class="identifier">socket</span> <span
            class="special">=</span> <span class="identifier">use_awaitable</span><span class="special">.</span><span
            class="identifier">as_default_on</span><span class="special">(</span><span
            class="identifier">tcp</span><span class="special">::</span><span class="identifier">socket</span><span
            class="special">(</span><span class="identifier">my_context</span><span class="special">));</span>
<span class="comment">// ...</span>
<span class="identifier">co_await</span> <span class="identifier">socket</span><span class="special">.</span><span
                class="identifier">async_connect</span><span class="special">(</span><span class="identifier">my_endpoint</span><span
                class="special">);</span> <span class="comment">// Defaults to use_awaitable.</span>
</pre>
    <p>
        In this example, the type of the <code class="computeroutput"><span class="identifier">socket</span></code>
        object is transformed from <code class="computeroutput"><span class="identifier">tcp</span><span
            class="special">::</span><span class="identifier">socket</span></code>
        to have an I/O executor with the default completion token set to <code class="computeroutput"><span
            class="identifier">use_awaitable</span></code>.
    </p>
    <p>
        Alternatively, the socket type may be computed directly:
    </p>
    <pre class="programlisting"><span class="keyword">using</span> <span class="identifier">tcp_socket</span> <span
            class="special">=</span> <span class="identifier">use_awaitable_t</span><span
            class="special">&lt;&gt;::</span><span class="identifier">as_default_on_t</span><span
            class="special">&lt;</span><span class="identifier">tcp</span><span class="special">::</span><span
            class="identifier">socket</span><span class="special">&gt;;</span>
<span class="identifier">tcp_socket</span> <span class="identifier">socket</span><span class="special">(</span><span
                class="identifier">my_context</span><span class="special">);</span>
<span class="comment">// ...</span>
<span class="identifier">co_await</span> <span class="identifier">socket</span><span class="special">.</span><span
                class="identifier">async_connect</span><span class="special">(</span><span class="identifier">my_endpoint</span><span
                class="special">);</span> <span class="comment">// Defaults to use_awaitable.</span>
</pre>
    <p>
        —<span class="emphasis"><em>end example</em></span>]
    </p>
</div>
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            <div class="copyright-footer">Copyright © 2003-2022 Christopher M. Kohlhoff<p>
                Distributed under the Boost Software License, Version 1.0. (See accompanying
                file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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            </div>
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